Structure and dynamics of two-dimensional colloidal crystals in confined geometry

Abstract

. The great advantage of colloidal systems are their convenient time (rnilliseconds) and length scajes (microns) which allow the observation of single particle trajectories and the variety of available interaction potentials.In this paper we present experimental data on the structure and dynamics of laterally confined 2D colloidal systems consisting of large superparamagnetic spheres lying on a smooth polymer substrate. A similar system has been already used by Zahn et al. [4] for the investigation of melting in large 2D systems. The lateral confinement is obtained by patterning the surface. An external magnetic field B perpendicular to the 2 0 plane is used to generate a magnetic moment M within the particles leading to a repulsive pair potential of the form V K M2/r3, where r is the distance between two particles.In this system the plasma parameter r, defined as ratio between the magnetic energy t ' and kBT, depends on both the external magnetic field B and the temperature T . For reasons of convenience, in this system usually temperature is kept constant and phase transitions are observed as a function of the magnetic field B. It is noticeable, that the well known interaction allowed a comparison of tbe experiment and theory without any free parameter! [4].The sample cell (Fig. la) is formed by two circular fused silica plates which are fixed at a distance of 1 rnm by an O-ring. To reduce the sticking of the particles, a smooth 3 4 p n thick film of poly(methy1-methacrylate) (PMMA) was spin coated on top of the bottom silica plate. To realize the lateral confinement we then applied thin structured copper foils onto the PMMA substrate. These foils which are commercially available in a variety of sizes and geometries as grids for Transmission Electron Microscopy (TEM), consist of a large number (2Ck100) of identical

Effect of confinement on the mode dynamics of dipole clusters

“…Additionally, most observed clusters had a coupling parameter that would refer to the solid state of a corresponding extended crystal. 5 In this phase, the diffusion coefficient of a colloidal dipole crystal scales only weakly with G, whereas signicant differences between angular and radial diffusion were found for clusters in hard-wall pots at low G. 25,45 In Fig. 7(a), angular diffusion D para…”

Binary colloidal systems in two-dimensional circular cavities: Structure and dynamics

“…15,20 The B-field dependence of the magnetic moment is well known to correspond to the Langevin function. For the magnetic fields as used in this work, however, the magnetic moment increases in a linear way with B.…”

“…For small numbers of particles, the system does not crystallize in a triangular lattice as known for infinite systems (Wigner crystal) [11][12][13][14] but the structure of finite clusters is dominated by the geometry of the confining boundary. 15 In circular cavities, e.g. the particles form a concentric shell structure.…”

Directed self-assembly of field-responsive fluids in confined geometries

“…[8][9][10][11][12][13] Furthermore, the study of confined self-assembly has revealed many interesting phenomena which depend on the nature of the confinement. [14][15][16][17][18][19][20][21] In the current study we will investigate the self-assembly of magnetorheological (MR) fluids in microfluidic confinement and the important role of channel topology.…”

“…In 2D cavities, the system boundaries define a finite closed area in which colloidal assembly takes place. 18,19 While this is a model system for studying the effects of closed boundaries upon self-assembly it is not practical for microfluidic applications. In 2D channels, the system remains open along one dimension and the colloids selfassemble between the two parallel closed boundaries defining the channel walls.…”